1. Field of the Invention
The present invention relates to a power switching inverter in a half bridge configuration that supplies high frequency AC power into a highly inductive load. This is done with a minimum number of components and almost nonexistent logic power in such a manner that the energy to turn off and on the power transistors is supplied proportional to the current flowing through them. The invention also relates to a simple method to draw the power from the line at a high power factor. The most significant application of this circuit is for a high frequency electronic dimming ballast for fluorescent tubes and other forms of gas discharge devices that are used to produce light either directly or indirectly. The concept also has value in the power inverter field for the generation of ultrasonic drive power used in industry for cleaning and welding and in medicine for many applications.
2. Brief Description of the Prior Art
Bridge inverters have been around for some time. Normally the power transistors are driven, either directly or through some form of base drive transformer. In this manner, the logic circuitry is able to create an adequate deadband to allow one transistor to turn off before the second one turns on. In the event an inductive load is to be driven, a pair of diodes are connected across the two transistors (one diode across the emitter collector of each transistor) in order to handle the currents that flow as a result of the inductive load when the transistors are driven out of phase. This method of drive control depends upon the transistor specification such that it will turn off within the time alloted for it to do so.
Utilizing a base drive transformer with the primary in series with the load current has been another practical method of reducing drive energy while properly driving the power transistor. However, this method of self drive requires that the load be resonant and the operating frequency is fixed by the resonant frequency of the load. Thus, up to now there has not been a method of utilizing the current delivered to the load as the source of drive power for the drive transistor while creating a variable frequency drive with the appropriate protection to prevent one transistor from coming on before the other one has completely turned off.
Practically all electric devices other than those used on transportation equipment or solar powered systems draw their energy from the AC power line. The supply voltage may vary depending upon the application and country, from around 100 up to 480 volts. The frequency will be either 50 or 60 Hertz. In order to operate a high frequency inverter I must first convert the input line power to direct current and filter it so that the circuit will not be subjected to a high degree of line frequency ripple. Unfortunately simple rectifying of the power line and filtering with a large capacitor produces a high degree of power line distortion normally characterized as poor power factor. Although this has not been a significant problem in the past because only a small amount of electric equipment used rectification compared to the total load, this problem is rapidly changing with the installation of large computer systems, computer terminals at every desk, and now solid state electronic lighting systems. By choosing the appropriate capacitor inductor network at the output of the conventional bridge rectifier, this problem may be solved and the power factor brought to 0.9+. It is also necessary to protect the diodes in the bridge rectifier from power line transients that can create voltages higher than the rate of blocking voltage of the bridge diode. In addition, noise generated within the inverter power supply circuit must be kept off the power line. It is the normal practice to install a balun type of transformer configuration on the two input leads. This than requires two magnetic elements in the input circuitry, the input baluns just described and the power factor correction inductor. As a further point in the prior art, any form of frequency control in the past utilizing a separate transistor drive has had to have a power supply of some nature associated with it in order to supply adequate power to drive the transistors. The amount required to do this has a substantial negative impact on the circuit's efficiency as well as requiring additional components. Present solid state electronic ballasts, although more efficient than the old style core and coil types are still too expensive and/or complex to be competitive in today's market.